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Quantitative Microbial Risk Assessment is a preventive strategy for microbial safety of drinking water prepared from surface water (see picture below) and implemented in Dutch Legislation. The major components is determining the source water quality and the presence of microbial pathogens, assessment of the Pathogen Elimination Capacity of the treatment and exposure assessment and risk calculations.


Quantitative microbial risk assessment versus fecal indicator monitoring

Water can contain microbial, chemical and radiological contaminants that can cause adverse health effects when consumed. Providing water that is safe for drinking and other intended uses is crucial to prevent diseases in the community. Chemical contaminants typically don’t occur at levels that cause acute health effects, but long term exposure can lead to health problems. Microbial contamination however can cause acute outbreaks of disease, even at very low levels of contamination. Infected persons and animals shed high numbers of pathogenic microorganisms in their feces. These can be viruses, bacteria, protozoa or helminths. Helminths are mostly an issue in hot climates and very poor water treatment, but the other three pathogens are also relevant for developed countries as they are generally found in domestic waste water. Key characteristics of relevant waterborne pathogens in the Netherlands are summarized in Table 1.

CHARACTERISTICS OF RELEVANT WATERBORNE PATHOGENS IN THE NETHERLANDSWastewater treatment has little effect on these pathogens, and therefore these pathogens are also found in surface waters affected by treated wastewater discharge. Livestock, wildlife, waterfowl and pets also contribute to contamination of surface water or other water sources. Ingestion of one or a few of these pathogens can already cause an infection, often leading to diarrhea and sometimes to more serious diseases (WHO 2011). Therefore microbial risks are the primary concern for safe water supply. The WHO (world health organization) promotes a risk based approach for drinking water supply, because water quality analysis only provides limited verification of drinking water safety (WHO 2011).

Microbial contamination with pathogenic viruses, bacteria or protozoa is relevant even below detection limits. Furthermore their occurrence can be highly variable, especially in small scale systems. Microbial water quality is tested for the absence of E. coli, an indicator bacteria present in high numbers in feces of warm blooded animals. Detection of E. coli is a clear indication of recent fecal contamination, however outbreaks of disease have occurred when E. coli was not detected. Therefore a routine water quality analysis doesn’t guarantee continuous safety. Besides routine monitoring, water companies in the Netherlands have the legal requirement to perform quantitative microbial risk assessment (QMRA) every three years (VROM 2005, Bichai and Smeets 2013).

*Bichai, F., & Smeets, P. W. M. H. (2013). Using QMRA-based regulation as a water quality management tool in the water security challenge: Experience from the Netherlands and Australia. Water Res, 47(20), 7315-7326. doi:

** VROM-Inspectorate. (2005). Inspectorate guideline; Assessment of the microbial safety of drinking water. the Netherlands.


QMRA starts by monitoring (or estimating) levels of pathogens in the source water taking into account the variability of contamination due to seasonality or events like CSO (combined sewer overflow) due to heavy rainfall. Then the removal of pathogens by drinking water treatment is estimated either by monitoring the removal of indicator organisms by the treatment system, or by using process models published in scientific literature. The Watershare treatment calculator provides access to the most recent insights in treatment efficiency and thus eliminates the need for literature review for each QMRA>


The removal of pathogens is expressed on a 10log scale, e.g. 2 10log equals 99% removal. Because viruses are very small they are poorly removed by filtration, and they can survive some levels of disinfection. Protozoa like Cryptosporidium are larger, but are not affected by chemical disinfection. Because the various pathogens pose different challenges to drinking water treatment, the risk is assessed for four index pathogens: enteroviruses, Campylobacter bacteria, Cryptosporidium and Giardia. The concentration of pathogens in drinking water is estimated by applying the estimated removal to the pathogen concentration in source water. From the drinking water consumption, the consumer exposure (dose) to pathogens is calculated. A dose-response relationship is used to calculate the risk of developing an infection at this level of exposure. Thus the daily risk of infection is calculated and from that the annual risk of infection. To incorporate variability and uncertainty in the risk estimate, a stochastic approach is used. Each element in the risk assessment is described by a probability density function and these are combined in a Monte Carlo simulation to estimate the risk (Schijven et al. 2011). In the Netherlands the theoretical risk of infection needs to comply with the legal requirement of one infection per 10.000 persons per year, which roughly equals a concentration of one pathogen in one million liters of water.

The QMRA Treatment Calculator

You enter some information on your treatment process, and on the pathogen or indicator you want to remove.
You then receive a complete overview of removals reported in the specialised literature, including the dispersion between and within the studies.



Models are available for a number of processes (slow sand filtration, UV, chemical disinfection). For these processes, you can input specific conditions or an overview is presented of the model parameters in the literature.

QMRA is a work in progress and a number of pathogen-process combinations still need to be built into it. The tool contains a list of the combinations that are currently operational.

We could use your assistance in the further development of the tool: we would like to incorporate your feedback and measurement results and thus broaden the tool’s practice-data base. So email us with your comments.

*WHO (Ed.). (2011). Guidelines for Drinking Water Quality, fourth edition World health organization, Geneva, Switzerland.


There are several forms of support available to ensure that you optimize your use of the QMRA Treatment Calculator, and that you are updated on all the latest developments.
Specifically, you can make use of the following options to suit your particular situation:

  • Support in using the tool to conduct a risk assessment (QMRA)
  • Evaluation of your treatment process to determine how its performance compares to that of others, on the basis of existing measurement data or of a specially designed monitoring programme.
  • Advice in determining the key factors that affect your treatment, so as to identify potential moments of risk or to improve the system.
  • Completing the tool with processes or pathogens that it does not yet include.
  • Access to QMRA-related projects or research.


Mentioned in: 2 Publications
Enrolled in: 1 Cases in 2 Countries

The QMRA Treatment Calculator tool draws on KWR knowledge which has been published:

  • Smeets, P. W. M. H., L. C. Rietveld, W. Hijnen, G. Medema, and T. A. Stenstrom.
    "Efficacy of Water Treatment Processes." Kiwa Water Research, 2006.
  • Hijnen, W.A.M., and G.J. Medema.
    "Elimination of Micro-Organisms by Water Treatment Processes."
    KWR Watercycle Research Institute Series. London: IWA Publishing, 2010.
  • Bichai, F., & Smeets, P. W. M. H. (2013).
    Using QMRA-based regulation as a water quality management tool in the water security challenge: Experience from the Netherlands and Australia. Water Res, 47(20), 7315-7326. doi:

Tool Expert(s)

Patrick Smeets

Senior scientific researcher, KWR

+31 (0)30 60 69 584

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